Effect of high energy ion irradiation on silicon substrate in a pulsed plasma device

We have performed an experimental analysis on the investigation of high energy ion beam irradiation on Si(1 0 0) substrates at room temperature using a low energy plasma focus (PF) device operating in methane gas. The surface modifications induced by the ion beams are characterized using standard surface science diagnostic tools, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photothermal beam deflection, energy-dispersive X-ray (EDX) analysis and atomic force microscope (AFM) and the results are reported. In particular, it has been found that with silicon targets, the application of PF carbon ion beams results in the formation of a surface layer of hexagonal (6H) silicon carbide, with embedded self-organized step/terrace structures.     

Time resolved studies on X-rays and charged particles emission from a low energy plasma focus device

The time resolved studies on soft X-ray, hard X-ray, electron beam and ion beam emissions from a low energy plasma focus device are carried out simultaneously by employing a photodiode X-ray spectrometer, a scintillator photomultiplier tube, a combination of Faraday cup and Rogowski coil assembly and a biased Faraday cup, respectively. The soft X-ray is seen to be emitted in short multiple pulses corresponding to different pinch stages where as it is a single for hard X-ray, which corresponds to only maximum pinch stage. Similarly, multiple pulses of electron beam is found, which also corresponds to different pinch stages and these pulses are analogous with the soft X-ray pulses. The effective hard X-ray photon energy is estimated by foil absorption technique and found to be around 110 keV, which is consistent with the observed electron beam energy distribution. The simultaneous investigation of the electron and ion beam shows that both are accelerated by the same local field generated during the pinching process. The detailed results of time resolved studies on various radiations are incorporated in this Letter.     

Improved performance and stability by an Al/Ni bilayer cathode in organic light-emitting diodes

Al/Ni bilayer cathode was used to improve the electroluminescent (EL) efficiency and stability in N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)-based organic light-emitting diodes. The device with LiF/Al/Ni cathode achieved a maximum power efficiency of 2.8 lm/W at current density of 1.2 mA/cm², which is 1.4 times the efficiency of device with the state-of-the-art LiF/Al cathode. Importantly, the device stability was significantly enhanced due to the utilization of LiF/Al/Ni cathode. The lifetime at 30% decay in luminance for LiF/Al/Ni cathode was extrapolated to 400 h at an initial luminance of 100 cd/m², which is 10 times better than the LiF/Al cathode.     

Self-organized transformation to polyaniline nanowires by pulsed energetic electron irradiation in a plasma focus device

An ingenious method for fabricating network of polyaniline nanowires at room temperature in microsecond timescale is demonstrated by using the pulsed electron beam of a plasma focus device. The electron beam of the plasma focus device having a wide range of energies (10-200 keV) was irradiated on to the freestanding polyaniline film. The growth of polyaniline nanowires on the surface of film sample is confirmed by field emission scanning electron microscope images showing nanowires of about 50-80 nm in diameter and up to few tens of micrometers in length.     

Crystallization of amorphous zirconium thin film using ion implantation by a plasma focus of 1 kJ

In this work preliminary results of amorphous zirconium crystallization using ion beam pulses are presented. Energetic argon- and oxygen-ion beams generated by a plasma focus device were used to promote crystallization on amorphous ZrO₂-2.5 mol% Y₂O₃ film deposited by chemical solution deposition onto silica glass substrate. The films were burnt at 370 °C for 1 h in normal atmosphere previous to plasma irradiation. The irradiation was performed by means of successive pulses of ion beams. The evolution of the surface morphology and crystallization was followed by AFM and X-rays diffraction in a grazing incidence asymmetric Bragg geometry (GIAB), respectively. Argon-irradiated films showed highly nucleated cubic zirconia after 10 pulses. On the other hand, oxygen-irradiated films showed a delayed and less extensive cubic nucleation, but a more ordered structure and well-defined grains.     

Radiographic method for measuring the continuum hard X-ray output spectrum of a Plasma Focus device

A radiographic method is proposed and then applied to infer the continuum part of the hard X-ray spectrum of a 4.7 kJ Plasma Focus from differential absorption measurements on metals. Copper, nickel, titanium and silver samples with thicknesses spanning between 0.1 and 10 mm were employed as filters. The X-ray radiation was detected using a standard radiographic screen-film system. The results show the presence of a dominant peak around 75 keV with significant spectral components in the range of 40 to 200 keV. The method is easy to follow, inexpensive, and allows for calibrated, single shot, spectral measurements.     

Relativistic electron beams detection in a dense plasma focus

Fast electron beams into a hollow anode of a small plasma focus machine (2 kJ, 4 μF) were measured. The diagnostic method designed for this purpose is founded in a small Rogowski coil introduced into a cavity performed in the anode. By means of this, electron beam pulses of about 10 ns width generated in the plasma focus are detected. Simultaneously, hard X-ray signals obtained from a scintillator-photomultiplier system are registered. The electron beam energy was measured through the time-of-flight of the electrons between probe and anode top. The beams are found to be relativistic and its energy is into the range of hard X-rays energy. An analysis of signal intensities and relative delays for three hundred shots are here presented. Received 28 February 2002 / Received in final form 7 May 2002 Published online 24 September 2002     

Dynamics of ablation plasma induced by a femtosecond laser pulse in electric fields

Direct observations of ablation plasma dynamics in electric field is presented. A time-resolved spatial profile of the ablation plasma induced by femtosecond laser ablation (fsLA) with high fluence is visualized using a planar-laser-induced fluorescence (P-LIF) method. The external electric field is produced by installing a mesh electrode at 6 mm from a Samarium solid target. The Sm ion plasma created by the fsLA showed collective motion regardless of the external electric field, until they reached close to the electrode. When the accelerating and decelerating field was applied, the ions almost disappeared behind the electrode from the field of view. The observations are understood utilizing a SIMION simulation with a conceivable potential gradient caused by Debye shield effect, which is that the ablation plasma keeps the same potential as the target voltage and follows electric potential gradient near the mesh electrode. It is also revealed that this effect degrades time-of-flight resolution at high fluence irradiation. This work gives a new direction for further developments of a fsLA time-of-flight spectrometer.     

Enhancement of X-ray emission in the side on direction in a Mather-type plasma focus

A 1.8 kJ Mather-type plasma focus (PF) for argon and hydrogen filling is examined. Two anode configurations are used. One is tapered towards the anode face, and the other is cylindrical but the face is cut at different angles. At optimum conditions, the system is found to emit Cu–Kα X-rays of about 1.6±0.1 J/sr in the side-on direction for argon filling, which is about 32% of the total X-ray emission. In 4π-geometry, maximum total X-ray yield and wall plug efficiency found are 26.4±1.3 J and 1.5± 0.1% respectively. The modified geometry may help to use the PF as a radiation source for X-ray diffraction.     

Influence of the photoionization process on the fragmentation of laser desorbed polycyclic aromatic hydrocarbons

Characterization of polycyclic aromatic hydrocarbons (PAHs) samples has been performed by laser desorption combined with multi-photon ionization technique using two different geometries of the ionization laser beam. This comparative study evidences the strong influence of ionization laser fluence on PAH fragmentation. Through a ∼10³ enlargement of the ionization probe volume and 10⁴ reduction of laser fluence over previous studies, fragment free mass spectra are obtained with higher sensitivity and selectivity. The ability to measure fragment free PAH mass spectra is a very important step in the end goal of measuring complex unknown mixtures of PAH desorbed from solid surface such as soot samples.     

Effect of interface states on negative capacitance characteristics in GaAs homojunction far-infrared detectors

Bias-, frequency- and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. The origin of this effect is believed to be due to the carrier capture and emission at interface states, and has been confirmed by a comparison study of capacitance characteristics on p-GaAs HIWIP detectors with different interface state densities. A fitting data based on charging–discharging current and the inertial conducting current model show good agreement with the experimental observations. Received: 15 November 1999 / Accepted: 20 April 2000 / Published online: 5 October 2000     

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1s_i → 2p_j (Paschen notations) within 570-617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s₅ → 2p₂) neon transition at low discharge current (<220 μA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s₅) and electrode sputtered atoms in ground state using their partial energy level diagram.     

Preparation of silicon carbide film by a plasma focus device

Silicon carbide (SiC) films were grown on the silicon (100) substrate by a 20 kJ Mather-type dense plasma focus device. The preparation method and characterization data are presented. X-ray diffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (SEM) and nano-indentor were employed for the characterization of the samples obtained at different axial position of 50 mm, 90 mm, 130 mm and 170 mm, respectively. Polycrystalline 3CSiC were obtained at the position of 90 mm and 130 mm from XRD and FTIR spectra. SEM image showed that the silicon carbide films obtained at the position of 90 mm are porous on surface layer. Nano-indentor indicates that the film obtained at the position of 130 mm has the highest mechanical hardness.     

Effect of the configuration of the discharge chamber of the plasma focus on the neutron yield

Preliminary probe experiments on the Filippov-type plasma focus with the energy E = 70 kJ and a current of about 1 MA show significant stray currents flowing near the insulator. To suppress them and optimize the discharge circuit, the main discharge chamber elements, i.e., the insulator, anode and cathode liner, were changed. As a result, a 30-fold increase in the setup neutron yield to Y _n = 5· 10¹⁰ neutrons per pulse was detected.     

Experimental study of the neutron emission mechanisms in a Filippov-type plasma focus device

The results of an experimental investigation of neutron emission characteristics in the Filippov-type plasma focus facility Dena (90 kJ, 25 kV, 288 µF) with D₂ + %1 Kr as working gas are presented. From the experimental results, one can conclude that both thermonuclear and nonthermonuclear mechanisms are always present in neutron production, but their contributions to the total neutron yield are strongly dependent on the initial pressure and discharge voltage. It has been found that at constant discharge voltages and low pressures the beam—target interaction mechanism plays an important role in the neutron production and with increasing the pressure, its role decreases and the importance of thermonuclear mechanism increases. Also at constant pressures, the contribution of beam—target interaction mechanism in neutron production decreases with increasing the discharge energy. The value of the index in the empirical neutron yield scaling law Y_n I E^/2 was found to be about 3.82.     

Depth Dependence of Tetragonal Distortion of a ZnO/Mg0.1Zn0.9O/ZnO Heterostructure Studied by Rutherford Backscattering/Channeling

Rutherford backscattering and channeling are used to characterize the structure of a ZnO/Mg_0.1Zn_0.9O/ZnO heterostructure grown on a sapphire (0001) substrate by rf plasma-assisted molecular beam epitaxy. The results show that the Mg_0.1Zn_0.9O layer has the same hexagonal wurtzite structure as the underlying ZnO layer, and the heterostructure has a good crystalline quality with χ_min =5%, which is the ratio of the backscattering yields of aligned and random spectra in the near-surface region. Using the channeling angular scan around an off-normal <12^-13> axis in the {101^-0} plane of both ZnO and MgZnO layer, the tetragonal distortion e_T, which is caused by the elastic strain in the epilayer, is determined. The depth dependence of e_T is obtained by using this technique. It can clearly be seen that the elastic strain rapidly decreases with the increase in thickness of the ZnO film in the early growth stage and becomes slightly larger in the region of the Mg_0.1Zn_0.9O layer.     

Neutron velocity distribution from a superthermal solid 2H2 ultracold neutron source

We have determined for the first time the velocity distribution of neutrons from a solid ²H₂ ultracold neutron (UCN) source. The spectrum rises sharply above 4.5m/s and has a maximum around 7m/s after transport in an 8m long guide. The number of neutrons in the UCN velocity range (< 7m/s) may be increased by a factor of two by placing the experiment 1m above the UCN source level.     

稠密等离子体聚焦装置的中子产额及其稳定性研究

叙述了稠密等离子体聚焦装置(DPF)作为中子发生器的工作原理,讨论了影响DPF中子产额及其稳定性的一些因素和改进措施。利用不同电极形状、不同绝缘气体的场崎变开关可以很好地提高聚焦装置的运行性能,使装置输出中子的起伏由过去的约两个量级降到目前的3倍左右。通过对腔内电极的清洁,在不更换气体的条件下。中子产额无明显下降。